The present invention generally relates to optical disc reproducing apparatuses, and more particularly to an optical disc reproducing apparatus having a speed-change reproduction mode.
Conventionally, it is known that a speed-change reproduction is achieved by forcibly changing at a predetermined rate a scanning position (or reproducing position) of an optical head which reproduces pre-recorded video signals from an optical disc. This forced change in the scanning position of the optical head will hereinafter simply be referred to as a "jump" in the present specification. When it is assumed that the optical disc is pre-recorded in a normal recording mode, it is possible to play the optical disc in an n-times forward high-speed reproduction mode by making the optical head jump (n-1) times in a forward direction per revolution of the optical disc. Similarly, it is possible to play the optical disc in an n-times reverse high-speed reproduction mode by making the optical head jump (n+1) times in a reverse direction per revolution of the optical disc.
On the other hand, as an example of the conventional optical disc reproducing apparatus, there is a known optical disc reproducing apparatus which employs three light beams (beam spots) to reproduce pre-recorded signals from the optical disc. Out of the three light beams, one is a main light beam which is a zeroth diffraction light obtained from a diffraction grating, and the remaining two are sub light beams for control which are positive first diffraction light and negative first diffraction light obtained by the diffraction grating. Out of these three light beams, the main light beam is irradiated on a track which is to be scanned. On the other hand, the two sub light beams are respectively irradiated on opposite sides of the track scanned by the main light beam, that is, each sub light beam is irradiated on an intermediate portion between two successive tracks. In addition, the two sub light beams are respectively positioned ahead and behind the main light beam in the scanning direction.
According to this optical disc reproducing apparatus, it is possible to reproduce the pre-recorded information signal from the track by the main light beam (zeroth diffraction light). It is also possible to detect a focus error signal by the astigmatism method using a reflected light of the main beam, and in addition, it is possible to carry out a tracking control of the optical head based on reflected lights of the two sub light beams (positive and negative first diffraction lights).
When this optical disc reproducing apparatus plays the optical disc in the speed-change reproduction mode such as a high-speed reproduction mode and a still picture reproduction mode, the three light beams jump but the relative positional relationship of the three light beams are maintained constant.
FIGS. 1A through 1C are plan views of the optical disc showing the tracks on an enlarged scale, and the tracks may be concentric tracks or track portions of a single spiral track. For example, a video signal is pre-recorded on each track constituted by a row of intermittent pits. FIGS. 1A through 1C each show portions of three such consecutive tracks TR1, TR2 and TR3. It will be assumed for convenience' sake that a beam spot M of the main light beam jumps from the track TR2 constituted by a row of intermittent pits 1 to the track TR1 constituted by a row of intermittent pits 2. The track TR3 is constituted by a row of intermittent pits 3.
First, as shown in FIG. 1A, the beam spot M of the main light beam scans the track TR2 while a beam spot S1 of a first sub light beam scans an intermediate portion between the tracks TR1 and TR2 at a position leading the beam spot M by a time T in the scanning direction and a beam spot S2 of a second sub light beams scans an intermediate portion between the tracks TR2 and TR3 at a position lagging the beam spot M by a time T in the scanning direction.
When the beam spot M jumps from the track TR2 to the track TR1, the beam spots M, S1 and S2 take the positions shown in FIG. 1B before reaching the positions shown in FIG. 1C. In FIG. 1B, the beam spot M scans the intermediate portion between the tracks TR1 and TR2 while the beam spot S1 scans the track TR1 at a position leading the beam spot M by a time T in the scanning direction and the beam spot S2 scans the track TR2 at a position lagging the beam spot M by a time T in the scanning direction. On the other hand, in FIG. 1C, the beam spot M scans the intended track TR1 while the beam spot S1 scans an intermediate portion between the track TR1 and a track (not shown) adjacent thereto at a position leading the beam spot M by a time T in the scanning direction and the beam spot S2 scans the intermediate portion between the tracks TR1 and TR2 at a position lagging the beam spot M by a time T in the scanning direction.
When the beam spot M jumps from the track TR2 to the track TR1, the reproduced signal from the optical disc switches from a reproduced video signal from the track TR2 to a reproduced video signal from the track TR1. However, in the state shown in FIG. 1B before the beam spot M reaches the track TR1, the beam spot M scans the intermediate portion between the tracks TR1 and TR2. No reproduced video signal is obtained from this intermediate portion because the beam spot M scans neither the track TR2 nor the track TR1. As a result, a noise is generated in the reproduced picture for a duration of approximately one horizontal scanning period to ten odd horizontal scanning periods of the video signal per jump.
The noise caused by the jump can be made inconspicuous in the reproduced picture by carrying out the jump within a vertical blanking period of the video signal. For this reason, the noise is not a problem when playing the optical disc in the still picture reproduction mode and the high-speed reproduction mode (for example, a two-times speed mode) which carries out a relatively small number of jumps per revolution of the optical disc. However, when the number of jumps per revolution of the optical disc increases in the high-speed reproduction mode (for example, a ten-times speed mode), it no longer becomes possible to guarantee the jump within the vertical blanking period because a time interval of the jumps is restricted by conditions such as the mass of an actuator which displaces the optical head. In other words, as the number of jumps becomes large per revolution of the optical disc, it becomes impossible to mechanically cope with the extremely short time interval of the jumps so that each jump is within the vertical blanking period. For example, in the ten-times speed mode, a jump may occur at a central portion of one field, that is, the central portion of the picture. In this case, there is a problem in that the noise caused by the jump appears conspicuously at a central portion of the reproduced picture.